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The Genesis Report IIA Underlying Principle of Cable Design

Preface

In 1995 we published the Genesis Report on loudspeaker
cable design. In that document we established design
principals, scientifically proved, which are as valid today as
they were 12 years ago. This longevity is an unequivocal
endorsement of both the quality and depth of that original
scientific research. The Genesis Report laid the
foundations on which we based all our subsequent
loudspeaker cable designs, from QED Original, though to
Profile, SA-XT, X-Tube and of course Genesis.

Genesis II embraces the modern A/V and digital world we
all now inhabit. It explains how many of the principles that
apply to analogue cable design are equally applicable to
this new digital age.

Introduction

Around the world, the debate continues amongst both audio and videophiles, as to which cable designs offer
the best performance.

QED, an established market leader in this field, has won many coveted awards for its high performance
audio and A/V cable products. Key to this success has been the application of strong engineering principles,
coupled with extensive test and measurement of both our own and competitor's products.

In this report, we re-visit some of the areas covered in QED's original 1995 ‘Genesis Report' and also
investigate in more detail the underlying principles of how cables work. Included are the various
fundamentals relating to signal transmission that are scientifically intriguing and also central to developing
cables that really do perform better, for both Sound and for Vision.

The Cable's Role

Ideally every cable should transfer a signal between two components with zero loss and distortion. However,
in the ‘real' world this is not possible, because subtle changes occur in the signal, which result in readily
perceived changes to both sound and video quality. The degree of change is determined directly by the
design of cable being used.

Maximising ‘real' world cable performance requires an understanding of the signal transmission process
and the engineering tools available, to ensure the signal arrives in the best possible condition.

Some Theory and Interesting Facts

Cables appear to be very simple components in both their construction and operation. However, once you
venture beneath the surface, things become a lot more interesting.

Conductors:

Metal conductors such as copper work very well. This is because their structure supports a huge number of
free electrons, which are not locked into the copper's atomic structure and are therefore free to move.

A metal conductor consists of metal ions arranged in a regular pattern called a crystal lattice with free
electrons moving in the spaces between the ions. The movement of free electrons is random and very fast,
but with a net effect of zero current flow. Due to thermal energy, free electrons move about randomly and
jump between atoms. There are an incredibly high number of free electrons available to move, in just 1cm3
there are approximately 8.47x1022 electrons. When a potential difference exists across the metal conductor
and a current flows, electrons drift along at a velocity dependent on the current, size and type of conductor.
The resistance of the conductor is due to collisions between the free electrons and metal ions in which some
of the kinetic energy possessed by the electrons is transferred to the ions. This causes the lattice to vibrate,
converting this electrical energy to thermal energy.

Copper in particular is an excellent conductor, having a high conductivity ( ) because its outermost electrons
have a largely free mean path, about 100 atomic spacings between collisions. The electrical resistivity ( ) of
the conductor is inversely related to this mean free path.
Silver has the lowest resistivity with Copper and Gold following closely behind. Copper is far more abundant
and therefore used more extensively than silver.

Unfortunately all metal conductors possess resistance which opposes the movement of electrons and
dissipates some of the available energy as heat. Resistive effects on system performance were covered
extensively in the 1995 Genesis Report on speaker cables, but equally apply to video cables (for example
those used for HDMI transmission) where a cables resistance will dramatically impact its ability to transfer
high frequency signals over a long distance.

Superconductors

Superconductors are conductors that have no electrical resistance and conduct electricity with zero loss,
thus once an electrical current has started to flow in a superconducting ring it will do so forever.
Superconductors are also perfectly diamagnetic (i.e. they repel a magnetic field); this property was
discovered in 1933 and is named the Meissner effect.

The difficulty with using superconductors is the very low temperature required for the material to super
conduct. At the time of writing the superconductor with the highest critical temperature ever recorded is
Mercury Barium Thallium Copper Oxide which has a critical temperature of 139ºK (-210ºF or -134ºC).
So it looks as if we will wait some time until a viable superconducting cable works at room temperature.

With all this emphasis on metal conductors one might assume that they do all the work, but is this really
the case?

How Does the Signal Travel and How Fast?

The first question one needs to ask is “How is the signal transferred from one end of the cable to the
other?” It is easy to imagine that the free electrons at the sending end of the cable travel at an extremely
high velocity to the receiving end. This seems plausible until you measure the speed at which electrons
travel in a conductor, this speed is called the drift velocity.

For a 1 metre interconnect it takes the free electrons around 100 seconds to travel from one end to the
other. This is a significant time and clearly we do not wait that long for music or video to appear, so
another transmission mechanism must be occurring.

Thankfully a Scottish mathematician and theoretical physicist named James Maxwell (1832-1879) provided an
answer through his work on electromagnetic waves. Signals do in fact travel down cables as an electromagnetic
wave and this wave does travel at a very high velocity. If the metal conductor were perfect virtually all
the energy would be transferred from one end of the cable to the other in the electric and magnetic
fields around the conductor.

The energy flux vector described by an English physicist John Henry Poynting (1852-1914) shows that the
energy flow and its magnitude is derived by considering conservation of energy. That is, the total energy in
the system is constant, and the fact that the magnetic field can do no work.

S = E x HWhere ‘S' (energy flux) = Electric field x Magnetic field

The electromagnetic wave consists of electric and magnetic fields that are spatially at right angles to each
other and to the energy flux or power flow which is along the axis of the cable (Figure 2).

Furthermore electromagnetic wave theory shows that when conductor pairs are configured, the majority of the
energy will propagate in the space between the conductors (Figure 1).

These fundamental principles apply to all types of signal transmission, whether low frequency Audio or
high frequency Video, Digital or Analogue, even AC mains transmission across the national grid, the signal
energy travels in the fields around the conductor and the free electrons only carry a small fraction of the
energy travelling along the cable.

Importance of Dielectrics

Once it is understood the space around the conductor carries the signal energy, it becomes obvious the
material in that space is a very important component of cable design.

The dielectric space around the conductor has a direct effect on the propagation of the electromagnetic
wave along the cable, particularly in terms of its speed, energy storage and energy loss. Dielectric loss
increases linearly with frequency, with energy being dissipated as heat. The speed of the signal, velocity of
propagation (v) can be shown to be:

At this point, it is interesting to look at the mechanism responsible for dielectric loss. In the non conducting
dielectric the electrons are bound to the atoms; however when an electric field is present, the orbit of the
electrons becomes distorted, as the negatively charged electrons become attracted to the positive conductor.
The movement of these electrons requires energy from the signal which is ultimately dissipated as heat within
the dielectric. The better the dielectric the easier the atoms (not electrons) move and the lower the resulting loss

Electromagnetic Fields and More on Resistance

Unlike the electric field, the magnetic field does radiate from within the conductor, rising to a maximum at
the conductor dielectric boundary (Figure 3).

We know that electromagnetic fields around the conductor radiate for some distance, which means
that objects and materials (other than the dielectric) also present within the electric and magnetic field will
have a direct effect on the propagation and power loss of the signal.

It is also worth considering at this point that the external field energy lost, due to the resistance of the
cable, results in a loss energy field travelling in the copper conductor. Importantly this energy propagates
through the copper at a much slower speed than the external field and is dependent on the frequency of the
signal. Electromagnetic fields travel most quickly through poor conductors, which is why radio signals travel at
the speed of light through outer space. As the conductivity increases, the speed of the signal decreases. If the
conductor were perfect (no loss) then the electromagnetic field would be entirely reflected when it reached the
conductor surface. Metals have a refractive index just like glass.

In the real world the majority of the external field is reflected, however a small fraction penetrates the
imperfect conductor where it is attenuated quickly. There is a characteristic penetration distance, which is
often referred to as the Skin Depth.

The depth of penetration is defined as:

The depth of penetration is defined as ∆Z=√(2/μoσω)Where: μo =Permeability, σ= Conductivity, ω=2πf(frequency)

It is therefore desirable to reduce the variation in field across the conductor's area to ensure that a uniform
current flow is maintained at all frequencies, otherwise the varying field patterns within the conductor act like a
selective filter. (The results of the tests in the 1995 Genesis report still hold true, however when developing very
low resistance cables skin depth needs to be considered, as it was for the design of the Genesis speaker cable)

The velocity of the signals propagation in the conductor is also dependent on the transmission frequency; the
higher the frequency, the faster the transmission and the lower the frequency, the slower the transmission.

These factors were instrumental in QED developing its X-TubeTM technology, which takes into account the
requirement to minimise resistance whilst simultaneously ensuring the conductor is not subjected to a wide
variation in penetration by the electromagnetic field. (The result being a clean and dynamic sound)

It is also important to note that negative cable effects go largely unnoticed when their performance is
measured using conventional test techniques, such as sinusoidal test tones. Transient response testing is
much more revealing and was used to test speaker cables in the original Genesis report. This technique can be
applied equally well to low level signal cables to objectively assess performance.

Cables and the Importance of Characteristic Impedance

This area is often misunderstood. For low frequency audio signals (20Hz to 20kHz) traveling over a short
distance, the characteristic impedance of the cable has virtually no impact on the energy transfer between
the transmitter (amplifier) and receiver (speaker), and therefore is not a consideration in speaker cable
design. However, this changes once the wavelength of the electromagnetic wave, traveling down the cable,
becomes shorter than the length of that cable. Designing a high performance cable that carries an RF
(Radio Frequency) video signal therefore requires considerable care to ensure that signal energy is not lost.

Characteristic impedance is not constant and is dependent on the frequency of the signal being transmitted.
At low frequencies the cables Resistance (R) and Conductance (G) define the impedance, whereas at high
frequencies it is the Inductance (L) and Capacitance (C).

The general equation for Characteristic Impedance (Zo) is expressed as:

The Characteristic impedance is determined by the physical dimensions of the cable's conductors, their
spacing and the relative permittivity (_r) of the dielectric.

Ideally, the cable should transfer the electromagnetic wave energy into the receiver, without any wave
energy being reflected back. If the wave is reflected, it will travel back along the cable and interfere with the
incoming signal. The effect of having mismatched characteristic impedances is readily observed using ‘Time
Domain Reflectometry' (TDR) to establish how well a cable is manufactured and how well engineered
(matched) in impedance are the connectors (Figure 4). In practice a very short energy pulse is sent down the
cable. Any change in impedance causes the electromagnetic wave energy to be reflected back. If the cable is
not terminated virtually all the energy is reflected back.

When designing interconnects for use with high frequency signals the connector impedance is
critical. Whilst developing the QED ‘TTV' range of aerial cables, TDR analysis showed that many so
called ‘high end' connectors did not have the correct characteristic impedance. This results in high levels
of energy being reflected back to the source. When signal levels are particularly low, as is the case with
antenna feeds, the outcome is poor picture quality and fewer stations on the channel listing!

Conductor Chemistry

Conductor chemistry is very important for the establishment of high electrical conductivity. The presence of
elements such as silicon, magnesium and phosphorus all decrease the electrical conductivity. The most
widely used copper is electrolytic touch pitch (ETP) copper, which consists of extremely high purity metal
and oxygen in the range of 100-650ppm (parts per million). Oxygen is used as an alloying element and also
as a scavenger in reacting with most of the impurities in the copper. Adding around 0.02% oxygen to ETP
copper increases its conductivity. Oxygen Free Copper is produced primarily for its ability to be heat treated
without embrittlement, for ease of use when welding and brazing. The acronym for the term, oxygen-free
high conductivity copper OFHC™ is a registered trademark of Phelps Dodge Specialty Copper Products.
OFHC is a highly refined grade of copper that contains almost no oxygen or other impurities.

Certified
oxygen free high conductivity copper contains a minimum of 99.99% copper making it the purest metal in
common use. The IACS (International Annealed Copper Standard) gives a percentage ranking for
conductivity. ETP coppers fall typically within the range 100-101.5% IACS and OFHC around 102.4% IACS.

In the Digital World

Jitter and Crosstalk

Jitter is a term used to describe the time variance by which a digital signal arrives at the receiver. All metal
cable links have a finite bandwidth which attenuates the high frequency components of the digital signal. A
transition delay occurs (that is the delay in which the receiver detects a ‘1' or ‘0') which varies depending on
the pattern of the digital signal. Digital equipment normally has a built in tolerance to jitter, however it is not
always effective, depending on the frequency of the incoming jitter. In such cases the cable induced jitter is
transferred to the recovered clock signal, as is the case with an SPDIF signal. For long cable lengths the
digital signal ‘eye' closes and reduces in size, ultimately resulting in a failure of the link (Figure 5).

When more than one signal channel is required to be transferred down a single cable as is the case with
digital video, ‘crosstalk' the unwanted transfer of energy from one channel to another via inductive or
capacitive coupling has to be considered. Individual screening of signal pairs can improve crosstalk
performance, but this can be offset by a reduction in the available bandwidth. The effect of this is a reduction
in the ‘Pulse Rate Time' or ‘Slew Rate' which degrades signal transfer performance. Various design
techniques can be used to further improve performance, such as increasing the ‘conductor pair twist rates',
ensuring that pairs have different twist rates to minimise coupling and the use of better performing
dielectric materials.

The Future

New technologies place increasing demands on the transfer of information between audio and video products.
To realise the very best performance, cables are required to carry data more effectively at ever increasing
frequencies. This means designers are constantly investigating new materials and configuration geometries to
obtain the desired performance objectives.

The emphasis today is undoubtedly on digital transmission, where higher speed signals quickly reveal the
limitations of poorly designed and constructed cables. Digital systems are sensitive to signal loss and
degradation, so cables will continue to play a vital role in maintaining audio and video quality.

Wireless technology has improved considerably in recent years bringing a big improvement in connection
convenience. Wireless however is always likely to lag behind cable in terms of bandwidth, making it less
suitable for the demands of very high performance applications. Optical cables are more likely to replace
conventional metal cables in the longer term and have already done so in many areas of telecommunications.

QED is continually researching new materials and constructions to ensure their products offer technically
advanced interconnectivity solutions. Measuring, listening and viewing the results of this research will continue
into the future.

Summary

The fundamental principal of electromagnetic signal transmission via a cable remains unchanged, whether
it is applied to audio or video signals and in both the digital and analogue domains. Whilst technology is
unquestionably advancing rapidly, the physics of transmission is constant. We can however, continue to
refine and improve our understanding.

It is vitally important that a cable’s characteristics are designed to match the specific application required, in
order to minimise losses and changes in the signal being transmitted. It is not sufficient to consider purely
the cable, the connectors must also be correctly designed to minimise electromagnetic energy being
reflected back down the cable. Changes in impedance due to manufacturing tolerances have to be
minimised. This is particularly important at RF frequencies.

The real world mechanisms, by which the electromagnetic signal loses its energy, and the subsequent
reduction in velocity of the signal energy propagating through the conductor and dielectric, are important
considerations when determining the conductor configuration. When designing cables for high performance
A/V systems, interaction between conductor pairs is more relevant than ever and requires careful attention
to ensure minimal signal corruption.

Lowering cable resistance and minimising impedance at high frequencies has proven to be beneficial, under
both technical and subjective assessment, applying equally to low frequency speaker cables as well as high
frequency video cables. Cable geometry, which is way the different materials are configured, is central to
achieving good cable performance. The best materials incorrectly configured, will never outperform a
correctly configured cable using lower grade materials. The best solution is of course to use optimised
geometry with high grade materials.

At QED, our understanding of the physics of what is happening within a cable enables us to make truly
informed decisions about cable design. We can devise intelligent solutions, which will effectively convey
audio and video signals in optimum condition.